The present invention relates to a level shift circuit, an amplitude expansion circuit, and a drive circuit using these circuits.
In recent years, pulse width modulation (PWM) drive circuits have been often used for drive control of motors and actuators. To sufficiently turn on a power switching element of a push-pull circuit as a drive-stage circuit at the final stage, a predrive circuit preceding the push-pull circuit is required to output a voltage sufficiently higher than the power supply voltage for the push-pull circuit. For this purpose, the predrive circuit is normally formed using MOSFETs sufficiently high in breakdown voltage. With an output voltage from the predrive circuit, the gate voltage of the power switching element connected to a motor or an actuator is controlled, and by this control, PWM drive of the motor or the actuator is realized.
A conventional PWM drive circuit will be described with reference to FIG. 5. A PWM drive circuit shown in FIG. 5 includes a level shift amplitude expansion circuit 51 and a push-pull circuit 52 as a drive-stage circuit. The level shift amplitude expansion circuit 51 includes a level shift DC power supply line 53 for supplying a voltage VG, p-channel MOSFETs 54 and 55, and n-channel MOSFETs 56 and 57. The push-pull circuit 52 includes a drive-stage DC power supply line 58 for supplying a voltage VX and n-channel MOSFETs 59 and 60 as power switching elements. FIG. 5 also shows a control circuit 61 for controlling the level shift amplitude expansion circuit 51 and the push-pull circuit 52, a control circuit DC power supply line 62 for supplying a voltage VDD, an inverter circuit 63, and a load 64 such as a motor and an actuator, which are provided outside the PWM drive circuit.
The circuit 51, which is a circuit normally called a level shift circuit, is herein referred to as the level shift amplitude expansion circuit for the following reason. According to the present invention to be described later, a circuit for shifting the absolute level of a voltage while keeping the amplitude thereof unchanged is called a level shift circuit, while a circuit for expanding the amplitude of the level-shifted voltage is called an amplitude expansion circuit, and these circuits are handled separately.
The operation of the PWM drive circuit having the configuration described above is as follows.
A pulse signal with the amplitude of VDD is input into the gate terminal of the MOSFET 56 from the control circuit 61, and an inverted signal of the VDD-amplitude pulse signal is input into the gate terminal of the MOSFET 57 via the inverter circuit 63. When the gate terminal of the MOSFET 56 receives the “H” level and the gate terminal of the MOSFET 57 receives the “L” level, the MOSFET 56 is turned ON and the MOSFET 57 is turned OFF. The “L” level is then input into the gate terminal of the MOSFET 55, to is turn ON the MOSFET 55. This allows the voltage VG of the level shift DC power supply line 53 to be supplied to the gate terminal of the MOSFET 54 of the level shift amplitude expansion circuit 51 and the gate terminal of the MOSFET 59 of the push-pull circuit 52, turning OFF the MOSFET 54 and ON the MOSFET 59.
To ensure that the MOSFET 59 as a power switching element is reliably turned ON, the value of the voltage VG of the level shift DC power supply line 53 must sufficiently exceed the sum (VX+VT) of the voltage VX of the drive-stage DC power supply line 58 and a threshold voltage VT of the MOSFET 59. Note that the value of the voltage VDD also must be high enough to provide a gate voltage with which the MOSFET 60 as another switching element can be reliably turned ON. In general, however, this requirement can be easily satisfied.
In formation of the PWM drive circuit having the configuration described above, the level shift amplitude expansion circuit must operate with a power supply voltage exceeding the voltage (VX+VT) so that the power switching element on the upper arm side of the push-pull circuit 52 can be brought into complete conduction for full-swing drive. For this purpose, the level shift amplitude expansion circuit must be constructed of elements having a breakdown voltage sufficiently exceeding the power supply voltage. This breakdown voltage includes a gate breakdown voltage. To increase the gate breakdown voltage, a gate oxide film must be made thicker, and this increases the ON resistance. Elements of the drive-stage circuit and the level shift amplitude expansion circuit may be formed monolithically to have gate oxide films of different thicknesses determined to satisfy the respective necessary minimum gate breakdown voltages, that is, the respective breakdown voltages according to the voltages VX and VG. By forming in this way, the areas necessary for these circuits can be minimum. However, this complicates the process and also increases the cost.
The drive-stage circuit and the level shift amplitude expansion circuit may otherwise be formed so that the both circuits uniformly satisfy a higher one of the breakdown voltages required for the these circuits, that is, the gate breakdown voltage required for the level shift amplitude expansion circuit. In this case, however, the power switching element of the drive-stage circuit has an excessively large breakdown voltage margin. This increases the ON resistance per unit area, and thus excessively increases the size of the power switching element required to attain the necessary ON resistance, resulting in poor area efficiency.